Traveling-wave tube with independent phase and amplitude control



Apnl 3, 1962 J. B. CICCHETTI ETAL I 3,028,597

TRAVELING-WAVE TUBE WITH INDEPENDENT PHASE AND AMPLITUDE CONTROL Filed May 4, 1959 2 Sheets-Sheet 1 CURRENT CONTROL 250 2000 ANODE VOLTAGE 32 RELATIVE PHASE SHIFT I000 DEGREES F/g. 3.

HELIX VOLTAGE 0 36 300 240 |so |2o 60 0 40 -|2o |eo 3.0 *7

4 300 Bdb g F /g. 4.

ANODE VOLTAGE HELIX VOLTAGE J. B. CICCHETTI ETAL 3,028,597 TRAVELING-WAVE TUBE WITH INDEPENDENT PHASE April 3, 1962 AND AMPLITUDE CONTROL 2 Sheets-Sheet 2 Filed May 4, 1959 UTILIZATION DEVICE COMPEN' 42/ G SATUR \40 GEN.

PROGRAMMER 8t CONTROLLER FOR GAIN PHASE 3x FREQUENCY PHASE (HELIX VOLTAGE CONTROL Hg. 7. \IIO SCAN X M TR CONTROL United States Patent 0 3,028,597 TRAVELING-WAVE TUBE WITH INDEPENDENT PHASE AND AMPLITUDE CUNTROL John B. Cicc'hetti and Jack Kliger, Los Angeles, Calif.,

assignors to Hughes Aircraft Company, Culver City,

Caliik, a corporation of Delaware Filed May 4, 1959, Ser. No. 810,960 7 Claims. {(1 343-'-100) This invention relates generally to electronic devices and particularly to a traveling-wave type amplifier system having mutually noninterdependent phase shifting and gain controls.

It is frequently desirable in traveling-wave tubes to control or alter the relative phase shifts experienced by the microwave signals which are propagated therethrough as, for example, for purposes of amplification or modulation. Similarly, it is frequently desired to control or alter the effective gain of the traveling-wave tube to control the magnitude of its output microwave signal.

Normally, the phase shifting control may be achieved by adjusting the direct current potential of the slow-Wave structure of the traveling-wave tube which consequently controls the velocity of the electron stream therethrough. The gain control function is achieved by adjusting the magnitude of the current in the electron stream by controlling the direct current potential on a current control electrode in the electron gun. Although these are the major effects, some degree of coupling exists between the controls so that variations in the direct current potential on the currentcontrol electrode aiiect the phase shift and, to a lesser extent, variations in the direct current potential of the slow-wave structure of the traveling-wave tube affect the gain. Heretofore, no appreciable degree of independence between these two functions has been achieved exceptby complex and impracticable means such as feedback types of control circuits.

A common example of a system which may utilize traveling-wave tubes having independent phase and gain control functions is a microwave antenna system. The achievement of very long range, high resolution radar beams which can be scanned electronically necessitates in many applications the use of antennas consisting of arrays of radiating elements, high power amplifier chains to feed these elements, and electronic means for controlling the scanning patterns to be used. Traveling-wave tube amplifiers, by virtue of their gain and power handling capacities, can provide the high power needed to feed the individual elements and, as pointed out above, may .be utilized at the same time to adjust the relative phases of the radiated signals and, therefore, to control the an tenna beam direction. In addition, for a chosen beam rection, the relative gains of the amplifiers may be controlled to realize any of several possible antenna beam shapes. In an ideal system, the beam-shaping and scanning functions should be independent of one another and, if possible, of the transmitted frequency. An arbitrary variation of frequency from pulse to pulse, for example, is effective against countermeasures and scintillation tading. The possibility of achieving. essentially independent phase and gain controls has already been mentioned. The inherently broadband nature of traveling-wave tube amplifiers, which is a consequence of the fact that they contain nondispersiveradio frequency circuits, also makes it possible to achieve an essentially frequencyindependent system. The gain through a traveling-wave tube amplifier remains relatively constant over a wide band of frequencies. Because the amplifier is nondispersive, the phase shift through the tube increases linearly with frequency, acondition which is necessary if the anenna beam angle is to remain constant as the frequency is changed.

Clearly, the operational flexibility of such a scanning system depends upon how independently and accurately the phase and gain control functions can be performed. The degree of accuracy and independence of the control functions becomes particularly critical as the desired range and resolution of the radiated beam are increased so that the required number ofantenna elements becomes very large. A practical antenna system for providing a beam one of two degrees wide, for example, requires the use of approximately one hundred or more radiating elements.

It is therefore an object of the present invention to provide a microwave amplifying device having independent phase and gain controls.

It is another object to provide a high power broadband microwave amplifier device having a broad range of independent phase and gain control functions.

It is another object to provide a traveling-wave tube amplifier having independent, or mutually noninterdependent, electron stream current controls and electron stream velocity controls.

it is another object to provide an antenna array feed system having independent beam shape and direction controls. I

It is another object to provide a high powered, amplifying, substantially frequency-independent antenna array feed system having independent radiating beam direction and shape controls.

t is another object to provide a broadband multielement antenna system wherein the phase and amplitude of signals at each element may be independently controlled with considerable. versatility to determine a desired instantaneous beam direction and shape.

It is another object to provide a high'powered travelingwave tube fed multi-element antenna array which may be frequency shifted without affecting the radiating beam direction and shape.

These and other objects are achieved by the present invention in the following manner.

In one form, in accordance with the invention, a traveling-wave tube amplifier component may be provided as in an antenna feeding system. A first voltage source is connected to the current control electrode of the traveiing-wave tube and a second voltage source is connected to the stream velocity control anode, for example, the helix of the traveiing-wave tube. In one form of the invention, a predetermined fraction of whatever voitage is applied to the current control electrode is added to the voltage which is applied to the stream velocity 'con- .trol anode. The amplifier may then be inserted in an antenna array feed system to amplify and'irnpress microwave signats from the transmitter onto a particular radiating eiement. A master programmer and controller unit is connected to the trasmitter and to the first and second voitage sources to control the phase shift of the travelingwave tube and thereby the direction of the radiated beam; the gain of the amplifier and thereby the shape of the radiated beam; and the frequency of the transmitter to select a desirable frequency of operation. By a proper choice of the range over which the control voltages may be varied, each of the three control functions are mutually noninterdependent.

Another feature of the invention is that it may be utilized in conjunction with frequency shifting in an antenna system having dispersive feeds between rows or columns of radiating elements to achieve electronically a second dimension of direction scanning.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts, and in which:

FIG. 1 is a schematic view of a traveling-wave tube having gain and phase shift controls;

FIG. 2 is a graph plotting gain in decibels versus current control anode voltage;

FIG. 3 is a graph plotting relative phase shift in degrees versus voltage on the helix;

FIG. 4 is a graph plotting anode voltage versus helix voltage with families of curves of constant gain and of constant phase shift;

FIG. 5 is a generalized block diagram of a travelingwave tube system constructed in accordance with the present invention;

FIG. 6 is a schematic diagram of a traveling-wave tube with independent gain and phase shift controls; and

FIG. 7 is an antenna array feed system constructed in accordance with the principles of the present invention.

Referring more particularly to FIG. 1, there is shown a traveling-wave tube 10 having a helical slow-wave structure 12 aligned coaxially with and supported by a glass envelope 14. At the lefthand end, in the drawing, of the envelope 14 is disposed an electron gun 16 which is adapted to pro'ect a stream of electrons along the axis of the helix 12 to a collector electrode 18. Input means 26 and output means 22 are provided as shown for coupling microwave energy onto and off from, respectively, the slow-wave structure 12. The velocity of the electron stream and the ratio of the circumference to the pitch of the helix are such as to permit a cumulative electromagnetic interaction between the electron stream and the microwave energy on the helix whereby the microwave energy is amplified.

A current control anode 24 in the electron gun 16 controls the magnitude of stream current and thereby the gain or amplification of the traveling-wave tube. The voltage on the control grid 24 is supplied by a gain control supply 26. The direct current voltage on the helix 12 determines the velocity of the electrons through the helix region. Since this velocity influences the phase velocity of the growing microwave signal on the stream, the helix voltage effectively controls the phase shift of the signal through the tube. The magnitude of the direct current voltage on the helix 12 is determined and supplied by a phase control voltage source 28 which is connected to the helix. The variations of gain and phase shift with anode and helix voltages, respectively, are plotted for a typical S-band traveling-wave tube in the graphs of FIG. 2, FIG. 3 and FIG. 4. In FIG. 2 the curve 3% illustrates that a change in the current control anode voltage from 50 or 60 volts to approximately 250 volts results in a substantially linear change in gain from approximately to decibels. In FIG. 3 the curve 32 illustrates that the phase shift through the tube decreases by 2000 degrees when the helix voltage is shifted from 500 to 1000 volts. The data for these curves were measured at a frequency of 3.0 kilomegacycles. Similar data have been measured, however, over a wide band of frequencies.

Referring to FIG. 4, the chart in Cartesian coordinates illustrates by the vertical grid members lines of constant helix voltage over the range from approximately 790 all) volts to approximately 900 volts for a typical travelingwave tube, such as the traveling-wave tube 10. The horizontal grid members illustrate lines of constant current control anode voltage over a range of approximately 220 to 310 volts. The curved lines labeled in decibels (db) illustrate lines of constant gain with appropriate control changes being made in the helix and anode voltages. Similarly, the substantially straight lines having a positive slope of approximately /3 and labeled in degrees from +420 to illustrate lines of constant phase shift while appropriate controls are exercised regarding the anode and helix voltages. Thus, in a single diagram the quantitative dependence of gain and phase shift in a traveling-Wave tube on both the anode and helix voltages are shown. Although these curves indicate the characteristics of an S-band traveling-Wave tube, as is well known, traveling-wave tubes can generally be scaled to operate in other frequency bands; the curves shown are indicative of What can be obtained in any frequency range for which a traveling-wave tube can be built. This frequency range extends from less than 30 megacycles up to beyond the K-band region. The curves of FIG. 4 illustrate that the phase shift properties of the tube are relatively uniform over the range of parameters indicated and that the helix voltage is approximately three times as effective in changing the phase as is the anode voltage, the effect being approximately 6 degrees of phase shift per volt of helix voltage change as compared to approximately 2 degrees per volt of anode voltage change. In other Words, if one were to operate the traveling-wave tube at constant phase shift, for example, along the slanted line labeled 0" the gain could be varied from less than 37 decibels to more than 48 decibels by changing the current control anode voltage from 220 volts to 310 volts. It may also be seen, however, that in order to stay on the 0 line the helix voltage must be changed from 850 volts to approximately 875 volts; that is, if constant phase shift is desired as the gain is varied, the helix voltage must be changed by approximately /3 of the change in the anode voltage.

Referring to FIG. 5, a system is shown in which a traveling-wave tube amplifier tube is utilized which permits an independence between the control functions of gain and phase shift. A traveling-wave tube 34 is connected at its input end to a microwave generator 36 and at its output end to a generalized utilization device 38. A compensator network 40 is coupled to the travelingwave tube amplifier for controlling its gain and phase shift. The lefthand portion 42 of the compensator network determines the gain. This part of the circuit is labeled G. The righthand portion 44, labeled 4:, of the compensator network controls the phase shift of the traveling-wave tube 34. The midportion 46 illustrates the compensator circuit itself which may be of the type discussed in detail below,.but in general it provides and ermits gain control or phase control independently of the other as by assuring operation along the lines of constant phase shift or constant gain of FIG. 4. A programmer controller unit 48 is connected to the network 40 and to the microwave generator 36 as a master control for programming in any predetermined manner the gain, the phase shift and the frequency of operation of the traveling-wave tube amplifier 34. As has been noted above, the gain and phase operation of the travelingwave tube may be independent of its frequency of operation over a relatively broad band of frequency.

Referring to FIG. 6, a practical example of a portion of the system of FIG. 5 is illustrated. A travelingwave tube 59 is fed at its input end by a waveguide 52 and at its output end by a waveguide 54. A slow-wave structure helix 56 is connected within the tube between the input and output points and is aligned coaxially with an electron stream projected therethrough by the electron gun 58. A focusing magnet 60 may surround the traveling-wave tube for purposes of focusing the electron stream to flow within the helix to the collector electrode 62. A gain control voltage source 64 is connected to a stream current control electrode 66 and a phase shift control voltage source 63 is connected to the helix 56. A tube power supply '54) provides the other operating voltages and currents for the traveling-wave tube. A voltage source 72 is shown separately connected to the collector electrode to maintain it somewhat more positive than the helix 56 in orderv to minimize the adverse effects of secondary emission from the collector electrode.

A voltage divider in the form of a potentiometer '74 is connected across the output of the gain control source 64 and the phase control source is returned to a point 76 thereof. it may be seen that in operation a predetermined i'raction of the gain control voltage is thus always added to the voltage of the phase control source 6%; hence, if the phase control source itself is maintained at a constant potential output, the direct current potential on the helix 56 will nevertheless vary by a predetermined fraction of whatever amount the current control electrode is shifted in voltage by the source 64. For example, if the source 64 is increased by 109 volts, and if the point '76 is adjusted to tap off /3 of the output of the source 64, the voltage of the helix 56 would be increased by approximately 33 volts. Relating this to the curves of operation shown in FIG. 4, it is seen, for example, that operation along one of the lines of constant phase shift results if such a fraction of the anode voltage used primarily to control gain is added to the helix voltage. For example, for operation along the constant phase shift line labeled 0, if the helix voltage source 68 is kept at 850 volts and approximately or volts is added linearly thereto as the anode voltage is raised from 220 to 310 volts, the will vary within the ll-decibel range shown and the incidental phase shift, caused by the fact that the constant phase shift line is not strictly linear, will be approximately only 1 degree. Without the compensation network the phase error would be more than 120 degrees. if, on the other hand, it is desirable to operate the tube at a constant amplitude of signal output but with variable phase setting, an anode voltage of approximately 200 volts or less may be used 7 giving a gain of approximately 37 decibels with a maximum gain variation of i.25 decibels as the phase is changed a full 360 degrees symmetrically about the. zero degree reference line. The phase shift is accomplished by an approximately volt excursion of the helix potential on each side of the 850 volt helix bias.

Referring to PiG. 7, an antenna array system is shown utilizing traveling-wave tube feeds. The array may utilize phase shifting between radiating elements, gain control at each radiating element and/or frequency shifting techniques either for frequencyohift scanning in a dimensi nal orthogonal with the phase-shift scan direction; or, in another arrangement, the entire system may be made frequency-independent. In the latter arrangement, the frequency may be shifted without attesting direction or shape of the radiated beam. Reference numeral indicates a system comprising a column or a row 32 of radiating elements, of which three, such as 84, and 36 are shown. Other subsystems 85, and 92 indicate additional networks like the subsystem 8d and each includes a row of radiating elements similar to the network comprising a gain control source 10-2, a phase shift control voltage source Hi4 and a compensator network 106 coupled to the sources 102 and 104. A transmitter 108 is coupled to the input of each of the travelingwave tubes so that the microwave energy transmitted by the transmitter is fed through the traveling-wave tubes to the radiating elements from whence it is propagated into space. A scan control unit 110 is coupled to the transmitter 108 for purposes of controlling the frequency of the energy from the transmitter in accordance with a predetermined program. The scan control unit is also coupled to the gain and phase voltage sources 102 and 194 of each of the traveling-wave tubes 96, 98 and 100. Thus, also in accordance witha predetermined program the phase shift and gain of each traveling-wave tube may be varied to control the antenna beam shape and direction. The scan control unit 110 is also coupled to each traveling-wave tube amplifier of the subsystems 88, 9t) and 92. Thus, the beam control is extended tothe other rows of radiating elements. The transmitter res similarly is coupled to the traveling-wave tubes of each of the other subsystems and if no dispersion exists between the transmitter and the various subsystems, the entire system may be frequency shifted without affecting the beam shape or the beam direction. As indicated previously, this is desirable in systems applications where a frequency of operation may be selected which is not subject to inter fering signals or atmospheric interferences. Further, and more generally, a frequency may be selected which provides the best echo signal from a particular object. On the other hand, if dispersive waveguides or other dispersive elements, indicated by the blocks 112, 114 and 116, are utilized between the various subsystems, the system as regards radiation from the various rows of radiating elements will be dispersive and may be shifted in direction of radiation in the dimension of the stacking of the rows of radiating elements by shifting the frequency of the transmitter 108. 7

What has been discussed in the preceding specification concerning antenna feed systems has been expressly related to transmitting systems such as an antenna coupled to a transmitter. All of what has been said relates with equal validity to receiving systems which have not been expressly discussed for the sake of simplification and clarity. For example, only minor and obvious changes need be made in the structure illustrated in FIG. 7 to make it a receiving system. The signals received at the antenna elements, for example, would be applied to the inputs of the traveling-wave tubes 96, '98 and which might be adapted for low-noise, high amplification instead of high power, as would usually be most desirable in a transmitting system. in many applications, the two systems may be combined so that one antenna array is coupled to both a transmitter and a receiver.

There has thus been described a traveling-wave tube amplifier system and antenna feed system in which the phase shift and gain of the traveling-wave tube amplifier may be controlled independently from each other. Further, the system may be made frequency-independent so that frequency shifting as a third control function may be achieved independently from the other control functions. The system disclosed may also utilize dispersive links to provide an antenna beam which may be scanned by frequency shifting in one dimension and scanned in an orthogonal dimension by phase shifting, the magnitude of amplified output of each of the radiating elements being independently controlled to provide a desired beam shape. Thus, the high gain and sensitive phase characteristics of traveling-wave tubes are utilized to provide an exceedingly versatile high powered radiating antenna.

What is claimed is:

1. A traveling-wave tube system comprising a broadband travelingwave amplifier tube, an electron gun including a current control electrode for propagating an electron stream along a predetermined path within said tube, means including an anode for controlling the velocity of the electronsof said stream, a first source of voltage signals coupled to said current control electrode for controlling the gain of said travelling-wave tube, a second source of voltage signals coupled to said anode for controlling the phase shift of said traveling-wave tube, and means for adding a predetermined substantially constant fraction of said voltage signals from said first source to those of said second source for providing independence between said gain controlling and said phase shift controlling.

2. A traveling-wave tube system comprising a broadband traveling-wave amplifier tube having an input terminal and an output terminal, an electron gun including a current control electrode for propagating an electron stream along a predetermined path within said tube, means including an anode for controlling the velocity of the electrons of said stream, a first source of voltage signals coupled to said current control electrode for controlling the gain of said traveling-wave tube, a second source of voltage signals coupled to said anode for controlling the phase shift of said traveling-wave tube, means for adding a predetermined constant, compensating fraction of said voltage signals from said first source to those of said second source for providing independence between said gain controlling and said phase shift controlling, a source of microwave signals coupled to an input terminal of said traveling-wave tube, a device for utilizing said microwave signals coupled to an output terminal of said travelingwave tube, and a programmer meter controller coupled to said microwave source, to said first source, and to said second source for controlling in a predetermined program mutually independently the frequency of said microwave source, the gain of said tube and the relative phase shift of said tube.

3. An electronic antenna scanning system comprising an antenna having a plurality of radiating elements arranged geometrically ln a predetermined array, a source of microwave signals coupled thereto, a plurality of electron discharge traveling-wave amplifier means interposed between said microwave source and ones of said radiating elements, gain control means connected to each of said traveling-wave means, phase shift control means connected to each of said traveling wave means, compensation means coupled to each of said traveling-wave means for providing independence between variations of gain and variations of phase shift in said traveling-wave means, and a scan control means connected to said microwave source, to said gain control means and to said phase shift control means for controlling the radiation beam pattern of said antenna.

4. An electronic antenna scanning system comprising an antenna having a plurality of radiating elements arranged geometrically in a predetermined array to permit control of antenna radiation beam shape and direction, a source of microwave signals coupled to each of said radiating elements, a plurality of traveling-wave amplifier tubes interposed between said microwave source and different ones of said radiating elements, electron stream current control means coupled to each of said travelingwave tubes, electron stream velocity control means coupled to each of said traveling-wave tubes, compensation means coupled between the velocity control means and current control means associated with each of said traveling-wave tubes, and a scan control and programmer coupled to each of said current control means and to each of said velocity control means for scanning and shaping said antenna beam in accordance with a predetermined program.

5. An electronic antenna scanning system comprising: an antenna having a plurality of radiating elements arranged geometrically in a predetermined array; 2 source of microwave signals coupled to each of said radiating elements; a plurality of traveling-wave amplifiers, individual ones thereof being interposed between said microwave source and individual ones of said radiating elements, each of said traveling-wave tube amplifiers including a broadband traveling-wave tube, an electron gun for propagating an electron stream along a path within said tube and having a current control electrode, an anode for controlling the velocity of said stream, a current controlling source of voltage signals connected to said electrode, a velocity controlling source of signals connected to said anode, and compensation means for adding a predetermined proportion of the magnitude of said signals of said current controllingsource to the signals of said velocity controlling source; and scan control means and programmer for controlling the radiation beam shape and direction connected to said microwave source for controlling the frequency of said microwave signals, and to each of said current controlling sources for controlling the gain of each of said amplifiers independently of their relative phase shifting and the frequency of said microwave source, and to each of said velocity controlling sources for controlling the relative phase shift of each of said amplifiers independently of their gain and the frequency of said microwave source.

6. A traveling wave tube system comprising a broadband traveling-wave amplifier tube having an input terminal and an output terminal, an electron gun within said tube for propagating an electron stream along a predetermined path and including an electron stream current control electrode, a slow-wave structure anode for propagating microwave energy in electromagnetic interaction with said electron stream and for controlling the velocity of the electrons of said stream, a first source of voltage signals connected to said current control electrode for controlling the gain throughout a range in the order of 21 decibels of said traveling-wave tube by varying the magnitude of said voltage signals throughout a range of from approximately to 270 volts, a second source of voltage signals coupled to said slow-wave structure for controlling the relative phase shift throughout a range of the order of 360 of said microwave signals by varying the magnitude of the voltage signals from said second source in the range from approximately 780 to 846 volts, means for adding a predetermined compensating fraction equal to approximately one-third of said voltage signals from said first source to those of said second source for providing a maximum phase error of i2 over said gain range of 21 decibels and a maximum gain error of :1 decibel over said phase shift range of 360, a source of microwave signals coupled to said input terminal of said traveling-wave tube, a utilization device coupled to said output terminal of said traveling-wave tube, and a programmer controller coupled to said microwave source, to said first source, and to said second source for controlling in a predetermined program mutually independently the frequency of said microwave source, the gain of said traveling vvave tube and the relative phase shift of said traveling-Wave tube.

7. An electronic antenna system comprising an antenna system having a plurality of radiating elements arranged alternately in a predetermined array; a source of microwave signals coupled to each of said radiating elements; a plurality of traveling-wave amplifiers interposed between said microwave source and individual ones of said radiating elements, each of said traveling-wave tube amplifiers including a broadband traveling-wave tube, an electron gun for propagating an electron stream along a path within said tube and having a current control electrode, a helix for propagating said microwave signals in energy exchange relation with said electron stream and for controlling the velocity of said stream, a. current controlling source of voltage signals connected to said electrode for varying the potential thereof throughout a range of approximately 200 volts providing an approximately ZI-clecibel range of gain, a velocity controlling source of signals connected to said helix for varying the potential thereof over a range of approximately 60 volts to provide an approximately 360-range of phase shift of said microwave signals, and compensation means for adding a fraction equal to approximately one-third of the magnitude of said signals of said current control source to those of the velocity controlling source, said range of gain being thereby provided with a maximum phase error of less than i2 and said phase shift range being provided With a maximum gain error of :1 decibel; and scan control and programmer means for controlling the radiation beam frequency, shape and direction and connected to said microwave source and to each of said current controlling sources for controlling the gain of each of said amplifiers independently from their phase shifting and frequency of said microwave source and to each of said velocity controlling sources for controlling the phase shift of each of said amplifiers independently of their gain and the frequency of said microwave source.

References Cited in the tile of this patent UNITED STATES PATENTS 

